1. Field of the Invention
This invention relates to an emergency and service brake actuator for a wedge type brake and, more specifically, one which utilizes a fail-safe emergency actuation member and is improved to include a service cylinder and piston configuration mounted between the emergency actuation member and the wedge of the brake.
2. Description of the Prior Art
There has heretofore been provided a number of emergency and service hydraulic brake actuators such as those disclosed in U.S. Pat. Nos. 3,232,175; 3,295,423; 3,462,986; and Re. No. 26,432. These actuators all include an emergency brake actuation member which is mounted on a tubular structure which extends from the brake and houses an extended end of the stem of the wedge of the brake therein. A primary cylinder of the emergency actuation member is mounted on the tubular structure and includes a primary piston mounted for sealed movement within the primary cylinder. The primary piston includes a rod which extends from the first side of the piston into the tubular structure to be operably connected to the extended end of the stem. A spring acts on the second side of the primary piston and tends to produce emergency actuation of the brake if it is not opposed by primary hydraulic fluid introduced to the primary cylinder at the first side of the primary piston.
In these combined emergency and service hydraulic brake actuators, the operable connection between the emergency actuation member and the wedge of the brake included a means for introducing sevice hydraulic fluid into the interior of the tubular structure. The service hydraulic fluid would act on the end of the rod and on the end of a stem of the wedge to selectively increase the relative distance therebetween during service actuation of the brake. This configuration required that the tubular structure be designed to withstand service hydraulic fluid pressure and that there be sealing provided to prevent the escape of the service hydraulic fluid into the emergency actuation member or the brake. Specifically, in order to separate the interior of the primary cylinder and the interior of the tubular structure, the rod from the primary piston was required to extend through a common wall therebetween and to be sealed to prevent the passage of hydraulic fluid from either interior into the other. In these prior art devices, the sealing around the rod was provided by one or more O-rings. However, it has been found that the O-ring seals in these and similar actuator configurations have not had the desired life expectancy needed for safe and reliable brake operation. Whether the decreased effectiveness of such seals is due to an inability to maintain proper tolerances for such an installation, the wear of such O-rings due to repeated axial movement of the primary piston rod, or other factors affecting the wear of the O-rings such as the requirement that opposite sides of the O-rings are subjected to different hydraulic fluid pressure levels is not fully known and might vary from configuration to configuration.
It should be noted that the prior art emergency and service actuators of the type described above must utilize similiar hydraulic fluids for both the emergency actuation and the service actuation functions. Since O-rings or other types of seals are made of materials which are selected depending on the type of hydraulic fluid to which they are exposed, if the seals are exposed to the hydraulic fluid from two sources, the same type of hydraulic fluid should be used for both sources. Using one type of hydraulic fluid may not be a problem with combined emergency and service brake actuators in some brake configurations. However, in some installations it is recognized that one type of hydraulic fluid might function well for an emergency actuator while another type of hydraulic fluid would be better utilized for service actuation.